Compact stripline low frequency band reject filter

ABSTRACT

A low frequency reject filter element is formed on a printed circuit board that utilizes the fact that power transmission in a waveguide is cut-off below a certain frequency. A quasi waveguide cavity is formed in printed circuit board using the top and bottom ground plane of the stripline circuit and using conductive via holes to form the side walls of the cavity. Waveguide cavity mode is launched from the input and output striplines by shorting them to ground. Transformers and matching via hole elements may be used to improve matching. The resulting filter is compact and is highly effective in suppressing low frequency transmissions.

This relates to filters in a stripline transmission line medium and in particular to filters on printed circuit boards in antenna arrays.

INTRODUCTION

Filters in a microwave system are used to suppress unwanted frequencies that may interfere with the functioning of the microwave system. In particular, an antenna designed to operate in a certain frequency band may be placed in an environment where many other antenna systems operating at different frequencies are transmitting powerful signals. This is often the case in a shipboard platform. Some communication antennae may be operating at Ku band while other long range surveillance radars may be operating at L and S band with very high transmit power. In order for this antenna to operate properly, the signals from the other lower frequency antennae received by this antenna must be substantially suppressed.

The distribution of RF signals in many modern microwave systems is through printed circuit boards because of their low design cost and low production cost. Filter elements are often integrated inside the circuit boards. Some of the filter elements used are parallel coupled filters, shunt stub filters, and coupled resonator filters. However, using these elements to design a high performance low frequency band reject filter often result in a filter that is physically large and is lossy.

I have conceived a better low-reject filter for on printed circuit boards that is relatively compact, low loss, and that rejects all frequencies below a certain cutoff frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an illustrative example compact stripline low frequency band reject filter;

FIG. 2 is an illustrative simulated performance graph of the example compact stripline low frequency band reject filter of FIG. 1 using Ansoft Corporation's High Frequency Structure Simulator software;

FIG. 3 is a front schematic view of the example compact stripline low frequency band reject filter as in FIG. 1;

FIG. 4 is a view from the perspective of an EM wave entering the example compact stripline low frequency reject filter inside the circuit board; and

FIG. 5 is a cross-section of the example compact stripline low frequency reject filter circuit board of FIG. 1.

DETAILED DESCRIPTION OF AN EMBODIMENT

Glossary. As used in this description and in the claims, the following terms shall be construed to have the following meanings:

(1) RF frequency bands are referred by their standard nomenclatures, as follows:

(a) “L-band” means 1-2 GHz;

(b) “S-band” means 2-4 GHz;

(e) “Ku-band” means 12-18 GHz;

(2) “antenna” means a structure for radiating or receiving electromagnetic waves;

(3) “low frequency” means frequencies below the operating frequencies of the antenna we want to protect;

(4) “cavity” means a metallic enclosure with two openings for RF energies to pass in and out;

(5) “quasi-cavity” means a structure that acts as a cavity without necessarily requiring a contiguously surrounding wall;

(6) “quasi-waveguide” means structure that acts like a waveguide without necessarily requiring a contiguously surrounding wall;

(7) “via-hole” means a hole formed in a printed circuit board and filled with a conductive material; and

(8) “cut-off” frequency means that frequency below which the signal is attenuated by a substantial amount (usually 10 dB or more, but in some applications 3 dB or more).

A compact stripline low frequency band reject filter can be formed in a printed circuit board to operate in a manner similar to the way prior waveguide cavities were used to filter out signals below a cut-off frequency of the waveguide. Transmission line tubes, for example, have been developed that include inner cavities forming an aperture having a characteristic frequency cut-off below which no electromagnetic radiation will pass through the cavity. FIG. 1 illustrates a low-cost, low profile method of creating a cavity in a printed circuit board for a compact stripline. The stripline circuit 12 includes striplines 13 and 14, first substrate 21 having a non-conductive portion 24 (FIG. 5) and a bottom ground plane 25 (FIG. 5), and second substrate 22 sandwiching the striplines 13 and 14. Second substrate 22 includes nonconductive portion 23 (FIG. 5) and top ground plane 26 (FIG. 5). A quasi waveguide is formed with the top ground 26 (FIG. 5) and bottom ground 25 (FIG. 5) of the stripline circuit and with the effective “side walls” formed by the via holes 17 and 18. Port 1 and Port 2 are the input and output ports of the stripline circuit. Striplines 13 and 14 are transformer sections that change the impedance of the striplines. Via holes 27 short the signal lines to ground, lauching a waveguide mode inside the quasi waveguide that couples the signal from Port 1 to Port 2.

Via-holes 15, 16, 19 and 20 on the printed circuit board short the top ground plane 26 to the bottom ground plane 25 to create a quasi-waveguide cavity between the two striplines 13 and 14.

FIG. 5 shows a side sectional view of the example filter element of FIG. 1, taken for example, along a plane intersecting the row of via-holes 18 in FIG. 1. The two striplines 13 and 14 are shown in the sandwich middle of the two nonconductive portions 23 and 24 of the substrates 22 and 21, respectively.

FIG. 4 shows a perspective view of the antenna element, as seen by the incoming electromagnetic guided wave 10. Incident signal will include the desired high-frequency signal, and may include interfering low-frequency signals. As the electromagnetic guided wave 10 propagates along the stripline 13, it passes through a transformer section to convert it to lower impedance. It is then shorted to the ground with via holes 17 and the stripline guided wave is converted into a waveguide mode inside the quasi-cavity created by the top ground 26 and bottom grounds 25 and the via holes 15, 16, 17, 18, 19, 20. RF signals having a frequency below the cut-off frequency of the quasi-cavity and will be rejected.

The via-holes 15, 16, 17, 18, 19, and 20 all short the top ground plane 26 to the bottom ground plane 25 to create the quasi-cavity shown in FIG. 4. They are plated holes which may be filled with conductive material to ensure good electrical contact. In its broadest sense, I have envisioned a quasi-cavity in the printed circuit board without regard to how the quasi-cavity is formed. Preferably, the quasi-cavity is formed by some effective number of via-holes spaced to create the quasi-cavity having the desired low-frequency cut-off value.

The size of the quasi-cavity is such that low frequency components of the quasi waveguide modes cannot be excited and are therefore rejected. My inventions include all natural extension of the aforementioned ideas by changing the number of via-holes, size of the via-holes, distribution of the via-holes in the cavity, and/or by modifying the transformer sections of the feed lines to change the pass band characteristics of the filter.

To illustrate my inventions, FIG. 3 shows one example (non-limiting) embodiment of quasi-cavity sizes. In it:

A=0.360″

B=0.306″

C=0.110″

H (not shown) is the stripline height of the striplines 13 and 14. H=0.020″.

D=0.40″

The middle portion of the striplines 13 and 14 are, as shown, 23 ohms. The narrower portions are, as shown, 50 ohms.

The substrate in FIG. 3 has a dielectric constant of 3 that works below 9 GHz.

Other sizes are contemplated by the inventions described herein. The size of the resultant quasi-cavity will vary as well depending on the frequencies desired to be rejected. In general, the example circuit of FIG. 3 can be scaled such that the low frequency rejection starts at different frequencies. The basic dimensions will be:

A=0.74 λg

B=0.63 λg

C=0.23 λg

D=0.05 λg

In the above equations, λg is the wavelength inside the substrate for the desired frequencies and the substrate height is 0.04 λg. The maximum frequency for the low frequency rejection band is 0.64 f, where f is the frequency of the beginning of the desired pass band. The rejection band is defined to be the frequency band with −20 dB or more rejection.

An electromagnetic simulation of the performance of a circuit described above is shown in FIG. 2. In it, port 1 is taken at the narrow end of stripline 13 and port 2 is taken at the narrow end of stripline 14. As seen in the simulation of FIG. 2, the quasi-cavity filter passes signals between 14 and 22 GHz, while rejecting by 15 dB or more all signals below 10 GHz. By evidence of simulation, it appears that quasi-cavities on printed circuit boards can reject low frequency signals while passing high frequency ones. The quasi-cavity is also an easily manufactured, non-bulky, and straightforward method of adding low-frequency rejection to a transmission stripline on a printed circuit board, without adding stubs, lumped elements, or surface components.

The cut-off frequency can be defined in many traditional manners. In the above example, signals are cutoff if rejected at 15 dB. Other applications may suggest an improved cutoff occur at a rejection of 20 dB. As shown in FIG. 2, even rejections at 3 dB can be envisioned to be an advantageous advancement in terms of ease of adding the filter to the printed circuit board versus degree of rejection obtained.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

1. A printed circuit board, comprising: a stripline to deliver electromagnetic energy; a transition region from the stripline to a quasi waveguide transmission line, and a quasi-cavity to filter electromagnetic energy passing through the stripline below a cut-off frequency.
 2. A printed circuit board according to claim 1, wherein the stripline is a first stripline and the board further includes: a second stripline to couple the electromagnetic energy from the first stripline.
 3. A printed circuit board according to claim 1, further including: a substrate mounting the stripline.
 4. A printed circuit board according to claim 1, wherein: the quasi-cavity includes a volume defined by a top ground plane, a bottom ground plane, and via-holes effectively defining side walls.
 5. A printed circuit board according to claim 4, wherein: the via-holes are spaced apart from each other at a distance related to the cutoff frequency.
 6. A printed circuit board according to claim 5, wherein: the top and bottom ground planes are formed on respective first and second substrates, and the quasi-channel is formed by via-holes in the first and second substrates that are electrically conductive with both the top ground plane of the first substrate and the bottom ground plane of the second substrate.
 7. A printed circuit board according to claim 1 wherein: a stripline signal is shorted to a ground to launch a quasi waveguide mode from a stripline TEM mode (transverse electromagnetic mode); and whereby the low frequency components of the quasi waveguide mode are reflected back when the low frequency components are below a cut-off frequency of the quasi cavity, resulting in low frequency rejection.
 8. A method of manufacturing a printed circuit board filter element, comprising: forming a conductive stripline on a printed circuit board, forming a transition from the stripline to a quasi waveguide transmission line, forming a quasi-cavity around the stripline on the printed circuit board tuned to filter out electromagnetic energy below a cut-off frequency transmitted by the stripline.
 9. A method according to claim 8, wherein the step of forming the conductive stripline includes the step of forming the conductive stripline on a substrate sandwiched between parallel ground planes.
 10. A method according to claim 8, further including the step of: electrically connecting the conductive stripline to one of the parallel ground planes, resulting in a transition from the stripline to a quasi waveguide transmission line.
 11. A method according to claim 8, wherein: the step of forming a quasi-cavity includes forming rows of conductive via-holes to surround the conductive stripline.
 12. A method according to claim 11, wherein: the step of forming the via holes includes: drilling holes in the printed circuit board and plating the holes.
 13. A printed circuit board, comprising: first and second substrates each defining an outer plane and an inner plane, the first and second substrates arranged together so their respective inner planes face one another; a first ground plane formed on the outer plane of the first substrate; a second ground plane formed on the outer plane of the second substrate; first and second conductive striplines sandwiched between the inner planes of the first and second substrates, each stripline having a terminal end proximate but not touching the other stripline; first and second rows of via holes arranged in a generally longitudinal direction of the striplines, the first row of via holes being arranged on one side of the striplines without touching the striplines and the second row of via holes being arranged on an opposing side of the striplines without touching the striplines, the via holes electrically connecting the first ground plane to the second ground plane; whereby the combination of (1) the first ground plane near the terminal ends of the terminal ends of the first and second conductive striplines, (2) the second ground plane near the terminal ends of the first and second conductive striplines, and (3) the via holes define a quasi cavity that prevents the passage of electromagnetic radiation below a cutoff frequency.
 14. A printed circuit board according to claim 13, wherein the printed circuit board is associated with a tuned antenna to receive an electromagnetic radiation signal of a particular frequency and the via holes are spaced apart about 0.05 times a wavelength of the electromagnetic radiation signal between the first and second substrates at the particular frequency. 